This invention relates to a hydraulic brake booster for automotive vehicles.
Because of their inherent advantages, hydraulic brake boosters are replacing the more conventional vacuum type boosters on certain automotive applications. A typical hydraulic brake booster design provides a spool valve, the position of which is controlled by the vehicle operator to establish a desired fluid pressure level in the pressure chamber of the booster to thereby effect a controlled brake application. Hydraulic brake boosters are capable of modulation; that is, the pressure level in the booster pressure chamber is responsive to changes of the operator-applied input force on the brake pedal. However, if the vehicle operator rapidly changes the pedal force, such as by fully applying the brakes and then partially releasing them, a situation may occur when the spool valve becomes unstable. Also, when the hydraulic brake booster is used in a braking system which includes a hydraulically actuated adaptive braking modulator, pressure impulses may be transmitted to the booster pressure chamber during cycling of the modulator. While these pressure impulses are so small that they cannot be felt by the vehicle operator, they do generate a hydraulic applying force on the spool valve, supplementing the operator-applied force on the spool valve. Although these pressure impulses may be very small in magnitude, their rapid occurrence sometimes causes an unstable condition in the spool valve.